The present invention relates, in general, to the field of an apparatus and method for accurately sawing a workpiece into two or more sections. More particularly, the present invention relates to an apparatus and method for cropping and/or slicing crystalline ingots, such as relatively large diameter polysilicon and single crystal silicon ingots, with great accuracy, speed and efficiency.
The vast majority of current semiconductor and integrated circuit devices are fabricated on a silicon substrate. The substrate itself is initially created utilizing raw polycrystalline silicon having randomly oriented crystallites. However, in this state, the silicon does not exhibit the requisite electrical characteristics necessary for semiconductor device fabrication. By heating high purity polycrystalline silicon at temperatures of about 1400 degrees, a single crystal silicon seed may then be added to the melt and a single crystalline ingot pulled having the same orientation of the seed. Initially, such silicon ingots had relatively small diameters of on the order of from one to four inches, although current technology can produce ingots of 150 mm (six inches) or 200 mm (eight inches) in diameter. Recent improvements to crystal growing technology now allow ingots of 300 mm (twelve inches) or 400 mm (sixteen inches) in diameter to be produced.
Once the ingot has been produced, it must be cropped (i.e. the "head" and "tail" portions of the ingot must be removed) and then sliced into individual wafers for subsequent processing into a number of die for discrete or integrated circuit semiconductor devices. The primary method for cropping the ingot is through the use of a bandsaw having a relatively thin flexible blade. However, the large amount of flutter inherent in the bandsaw blade results in a very large "kerf" loss and cutting blade serration marks which must then be lapped off.
At present, there are two primary techniques for slicing an ingot into wafers: the ID (inner diameter) hole saw and the slurry saw. The former is used predominantly in the United States in order to slice single crystal silicon and is so named due to the fact that the cutting edge of the blade adjoins a centrally located hole at its inner diameter in an attempt to reduce the flutter of the blade and resultant damage to the crystalline structure. Among the disadvantages inherent in this technique is that as silicon ingots increase in diameter, the ID hole saw must increase to three times the ingot diameter to allow it to cut all the way through the ingot to a point at which it becomes unwieldy if not unworkable.
As previously mentioned, an alternative technique also utilized in the United States but used primarily in the Pacific Rim countries is the slurry saw. The slurry saw comprises a series of mandrels about which a very long wire is looped and then driven through the ingot as a silicon carbide or boron carbide slurry is dripped onto the wire. Wire breakage is a significant problem and the saw down time can be significant when the wire must be replaced. Further, as ingot diameters increase to 300 mm to 400 mm the drag of the wire through the ingot reaches the point where breakage is increasingly more likely unless the wire gauge is increased resulting in greater "kerf" loss. Importantly, a slurry saw can take many hours to cut through a large diameter ingot.
As is the case with the ID hole saw technique as well, excessive "kerf" loss results in less wafers being able to be sliced from a given ingot with a concomitant greater cost per wafer. Moreover, the score marks of the ID hole saw and less than even cutting of the slurry saw wires result in an increased need for lengthy and expensive lapping operations to make the surfaces of the wafer smooth and parallel as well as to remove other surface markings and defects. This excessive lapping also requires even greater amounts of silicon carbide and oil or aluminum oxide slurries, the ultimate disposal of which gives rise to well known environmental concerns.
Laser Technology West, Limited, Colorado Springs, Colo., a manufacturer and distributor of diamond impregnated cutting wires and wire saws, has previously developed and manufactured a proprietary diamond impregnated wire marketed under the trademarks Superwire.TM. and Superlok.TM.. These wires comprise a very high tensile strength steel core with an electrolytically deposited surrounding copper sheath into which very small diamonds (on the order of between 20 to 120 microns) are uniformly embedded. A nickel overstrike in the Superlok wire serves to further retain the cutting diamonds in the copper sheath. The technique of cutting fixed workpieces with a direction reversing diamond wire is one that has been utilized, to date, primarily in a laboratory environment and not in a production process due to the inherently very slow cutting speed involved.